Purple Loosestrife
Lythrum salicaria
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A perennial with erect stems, often growing 6 to 8' tall, usually associated with moist or marshy sites. Leaves are simple, entire, and opposite or whorled. Lance-shaped leaves have smooth margins. Rose-purple flowers having 5 to 7 petals arranged in long vertical racemes.
This introduced species often escapes to aquatic sites such as streambanks or shorelines of shallow ponds.